The present invention is particularly adapted for use in conjunction with torque transfer devices in the nature of interleaved, multi-plate, brake packs that are actuated by a combination of mechanical and hydraulic means.
Torque transfer devices in the nature of such multi-plate, brake packs are often employed in conjunction with individual transmission output shafts, or the axle assemblies connected thereto. Such brake packs are generally subjected to axial compression in order to effect the desired braking action in response to depression of the brake pedal. That is, compression of the multiple, interleaved torque plates, with frection disks disposed therebetween, effects the torque transfer between the rotating output shaft and a fixed member secured to the vehicle. It is this torque transfer which actually slows the vehicle.
Combined mechanical and hydraulic actuation of braking systems have been available for many years. The combination advantageously actuates the brake mechanism with relatively modest foot pressure. However, when hydraulic actuation, and/or self energization is employed, the braking system is particularly susceptible to feed-back that might be erroneously interpreted. That is, the braking pressure applied by the vehicle operator, at least as the vehicle operator's foot receives feed-back which reflects the pressure required to effect the braking operation, appears to be disproportionately less than the operator might expect for the braking action desired.
Accordingly, when hydraulic actuation is initiated--particularly if the system initiates hydraulic actuation prior to mechanical actuation--brake pedal effort drops so rapidly that the operator is unable to respond with the required reduction of foot pressure to the brake pedal. In fact, the operator typically responds to the diminished tactile feed-back by applying even greater pressure to the foot pedal, thereby applying an even greater braking force. Under the circumstances of attempting to slow, or to bring the vehicle to a stop, the significant, instantaneous reduction of foot pedal resistance is interpreted as at least a partial loss of braking ability, which explains the operator'normal reaction of applying an even greater amount of pressure to the brake pedal.
The foot pressure applied to stroke, or depress, the brake pedal during the initial actuation of the braking operation is progressively resisted by the prior known brake pedal return spring, as represented by portion 10 of the curve depicted in FIG. 1. After the mechanical apply lever assembly, or the apply shaft, has rotated approximately eight degrees in prior art systems, hydraulic actuation of the braking operation is initiated. The tactile feed-back pressure thereupon drops, as reflected by portion 11 of the curve depicted in FIG. 1, to the extremely modest level of resistance afforded by a regulating spring that is customarily incorporated in the hydraulic brake apply valve. This low level resistance continues through virtually the entire rotational range of the apply lever, or shaft, as represented by portion 12 of the aforesaid curve.